A variable fuel injection rate type is known as one of the types of an in-line type fuel injection pump in a diesel engine. The fuel injection pump of this type has its plunger prestroke adjusted by changing the axial position of a control sleeve with respect to the plunger. This fuel injection pump is each of Japanese Patent Laid-Open Nos. 123756/1986 and 218769/1986, Japanese Utility Model Laid-Open No. 117981/1989, and Japanese Utility Model Publication No. 35727/1986.
More specifically, in the fuel injection pump of the aforementioned type, there is slidably arranged in a barrel a plunger, on which is fitted relatively movably a control sleeve. The plunger has its upper end facing an upper fuel compression chamber and formed at its center with an axial bore which extends in the axial direction. Moreover, the plunger is formed in its outer circumference with a longitudinal groove and a lead (i.e., inclined groove) intersecting with the former groove. This longitudinal groove has communication with the axial bore via a radial bore. On the other hand, the control sleeve is formed with a radial spill port.
In this fuel injection pump, no fuel is compressed for after the plunger begins its lift and before the longitudinal groove has its lower end edge shielded by the control sleeve. This time period is the "prestroke".
This prestroke can be changed by displacing the control sleeve in the axial direction of the plunger by the control rod. A time period after the prestroke and before communication between the lead and the spill port is the pumping effective stroke, for which the fuel is pumped. The pumping effective stroke can also be changed by turning the plunger relative to the control sleeve. On the other hand, if the lead and the spill port are aligned in the circumferential direction, there is established a non-injection state, in which the fuel is not compressed in the least by the plunger.
Because of the characteristics described above, the variable injection rate type fuel injection pump tends to be widely used. For expected effects, however, the position and size of the lead of the plunger have to be highly accurate. This high accuracy is difficult to achieve in the prior art.
Specifically, the ordinary in-line type fuel injection pump finds it relatively easy to reduce the dispersion of the fuel-pumping effective stroke because what the barrel undergoes is the vertical reciprocations of the plunger. In the variable injection type fuel injection pump, however, the control sleeve is moved up and down relative to the plunger. As a result, both the length from the lower end of the control sleeve to the edge of the spill port and the length L.sub.2 from the lower end of a port hole 10 of a plunger 10, as shown in FIG. 7, to the branching starting portion of the lead are effective to cause the dispersion of the fuel-pumping effective stroke. Moreover, a longitudinal groove 11 and a lead 11 have to be formed not at the upper end of the plunger 1 but at predetermined lower distances than the upper end. Thus, the lead has found it so seriously difficult to machine in the normal position that its positioning accuracy has failed to improve.
More specifically, the plunger has a basic structure shown in FIG. 8-A by way of example. Below a plunger body 1a having a predetermined external diameter and across a neck portion 1b, there is formed a face portion 1c, below which is formed a bottom end 1e across a neck portion 1d. The bottom end 1e is in abutment against the not-shown cam through a tappet so that the plunger reciprocates along the contour of the cam. Incidentally, the face portion 1c is so engaged by an injection rate adjusting sleeve other than the aforementioned control sleeve that its turning motion is regulated by the adjusting sleeve.
In the prior art, the following process is taken for machining the plunger to form the aforementioned lead. A plunger blank 100, which has been worked to the state shown in FIG. 8-A (i.e., to the state at which it has not been hardened yet), is machined on the basis of information inputted in advance to an NC machine, to form both an axial bore 13 having a desired depth from the upper end face and a round port hole 10 in a predetermined circumferential position.
Then, the plunger blank 100 is hardened to have its hardness increased. After this hardening step, the length L.sub.3 from the lower end of the port hole 10 to the bottom face 1f of the bottom end 1e is measured, as shown in FIG. 8-B, and is classified according to the difference from a reference size. This is because the hole position will disperse due to not only a machining error before the hardening step but also a deformation at the hardening step.
Next, the plunger blank 100 is chucked by the machine tool, and the programmed numerical value or coordinate of the length L.sub.3 inputted in advance to a NC machine 2 is corrected according to the aforementioned classification, a shown in FIG. 8-C. As shown in FIG. 8-D, moreover, the machine body and its machining head 3 are moved relative to each other with reference to the machining reference plane of the bottom face 1f of the plunger bottom end in accordance with a control command having correcting program data, thereby to machine the longitudinal groove 11 and the lead 12.
According to this machining process, however, the machining reference is located at the plunger lower end (or its bottom face). Hence, the length L.sub.2 (as will be called the "effective stroke size") from the lower end of the port hole 10 to the branching starting portion of the lead 12 will involve the dispersion of the length L.sub.3 from the lower end of the port hole 10 to the bottom end face 1f. Even if, however, the length L.sub.3 from the lower end of the port hole 10 to the bottom end face 1f is classified, the measurement itself of the length prior to the classification will involve minute errors. This makes it unavoidable to deteriorate the accuracy of the aforementioned effective stroke size L.sub.2.
As a result, in the prior art, the effective strokes disperse so seriously among the plunger individuals that the individual engine cylinders have irregular fuel injection rates and timings. Thus, the expectations of the fuel injection pump such as improvements in the output power and the mileage and reductions in exhaust emissions are hardly achieved.
In the process of the prior art, moreover, many troubles and steps have to be involved for measuring the length L.sub.3 from the lower end of the bottom hole 10 to the bottom end face 1f. Many steps also have to be involved for inputting different correction values to the NC machine in according to the classifications at the time of machining the lead. This makes it unavoidable to drop the machining efficiency as a whole.